Intermodulation byproduct cancellation in one or more nodes of a distributed antenna system

ABSTRACT

One embodiment is directed to a distributed antenna system (DAS) comprising a plurality of nodes, including a head-end unit and a plurality of remote units that are communicatively coupled to the head-end unit. The head-end unit is configured to receive uplink received signals from remote units that wirelessly transceive signals in a coverage area. The head-end unit is configured to sum two or more of the uplink received signals to produce a summed uplink received signal. At least one of the nodes of the DAS includes a processing device configured to determine a transfer function and apply the transfer function to signals in the DAS to cancel, reduce, attenuate, or eliminate intermodulation byproducts in the summed uplink received signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of PCT ApplicationSer. No. PCT/US2016/020570, titled “INTERMODULATION BYPRODUCTCANCELLATION IN ONE OR MORE NODES OF A DISTRIBUTED ANTENNA SYSTEM,”filed on Mar. 3, 2016, which claims priority to, and benefit of, U.S.Provisional Application No. 62/128,189, titled “INTERMODULATIONBYPRODUCT CANCELLATION AT A HEAD-END UNIT OF A DISTRIBUTED ANTENNASYSTEM,” filed on Mar. 4, 2015, the contents of all of which are herebyincorporated by reference in their entirety.

BACKGROUND

This disclosure relates to certain aspects and features of systems andmethods for reducing self-defense in the receive bands of atelecommunications system. One example of a telecommunications system isa distributed antenna system (DAS). A DAS can be used to extend wirelesscoverage in an area through the use of one or more repeaters andmultiple remote units coupled to each repeater. Head-end units can becoupled to one or more base stations that can each manage wirelesscommunications for different cell sites. A head-end unit can receivedownlink signals from the base station and distribute downlink signalsin analog or digital form to one or more remote units. The remote unitscan transmit the downlink signals to user equipment devices withincoverage areas serviced by the remote units. In the uplink direction,signals from user equipment devices may be received by the remote units.The remote units can transmit the uplink signals received from userequipment devices to the head-end unit. The head-end unit can transmituplink signals to the serving base stations.

SUMMARY

One embodiment is directed to a distributed antenna system (DAS)comprising a plurality of nodes, including a head-end unit and aplurality of remote units that are communicatively coupled to thehead-end unit. The head-end unit is configured to receive uplinkreceived signals from remote units that wirelessly transceive signals ina coverage area. The head-end unit is configured to sum two or more ofthe uplink received signals to produce a summed uplink received signal.At least one of the nodes of the DAS includes a processing deviceconfigured to determine a transfer function and apply the transferfunction to signals in the DAS to cancel, reduce, attenuate, oreliminate intermodulation byproducts in the summed uplink receivedsignal.

Another embodiment is directed to a method performed in a distributedantenna system (DAS) comprising a plurality of nodes, include a head-endunit communicatively coupled to a plurality of remote units. The methodcomprises transmitting a test reference signal from the remote unit,receiving a feedback signal from the test reference signal at a receiveantenna of the remote unit, and calculating, from the test referencesignal and the feedback signal, a transfer function to cancel, reduce,attenuate, or eliminate intermodulation byproducts in subsequent signalsreceived by the head-end unit. The method further comprises applying thetransfer function to signals of the DAS in order to cancel, reduce,attenuate, or eliminate intermodulation byproducts.

DRAWINGS

FIG. 1 is a block diagram of one exemplary embodiment of a distributedantenna system.

FIG. 2 is a flow diagram of one exemplary embodiment of a method forcalculating a transfer function designed to cancel, reduce, attenuate,or eliminate unwanted intermodulation byproducts received at thehead-end unit.

FIGS. 3-8 show various examples of how nodes in a distributed antennasystem can be configured for cancelling, reducing, attenuate, oreliminating unwanted intermodulation byproducts.

DETAILED DESCRIPTION

FIG. 1 depicts an example of a DAS 100 according to aspects and featuresof the subject matter described herein. The DAS 100 can include anetwork of various nodes including spatially separated remote units 102a-d communicatively coupled to a head-end unit 104 for communicatingwith one or more base stations 106 according to one aspect. For example,remote units 102 a-d can connect directly to the head-end unit 104. Inother aspects, the head-end unit 104 can be coupled to remote units 102a-d via a transport expansion unit or another intermediate device. Theremote units 102 a-d can provide wireless service to user equipmentdevices positioned in respective geographic coverage zones 110, 112.

The head-end unit 104 can receive downlink signals from the base station106 and transmit uplink signals to the base station 106. Any suitablecommunication link can be used for communication between base stationsand head-end units, such as (but not limited to) a direct connection ora wireless connection. A direct connection can include, for example, aconnection via a copper, optical fiber, or other suitable communicationmedium. In some aspects, the head-end unit 104 can include an externalrepeater or internal RF transceiver to communicate with the base station106. In some aspects, the head-end unit 104 can combine downlink signalsreceived from different base stations. The head-end unit 104 cantransmit the combined downlink signals to one or more of the remoteunits 102 a-d.

Remote unit 102 a can provide signal coverage in a coverage zone 110 bytransmitting downlink signals to mobile communication devices in thecoverage zone 110 and receiving uplink signals from the user equipmentin the coverage zone 110. In another aspect, multiple remote units 102b-d can provide signal coverage in a single coverage zone 112. Theremote units 102 a-d can transmit uplink signals to the head-end unit104. The head-end unit 104 can include a summer device to combine uplinksignals received from remote units 102 a-d for transmission to the basestation 106

A downlink signal transmitted by the base station can be repeated by thehead-end unit such that copies of the downlink signal are provided tothe remote units connected to the DAS. The remote units can then providethe copies of the common downlink signal to user devices in respectivecoverage zones.

The DAS 100 (and the nodes thereof including the head-end unit 104 andthe remote units 102 a-d) can be configured to distribute wirelesssignals that are communicated using a wireless air interface thatsupports one or more of frequency-division duplexing (FDD) and/ortime-division duplexing (TDD). Also, the DAS 100 can be configured todistribute wireless signals that are communicated using a wireless airinterface that supports one or more of themultiple-input-multiple-output (MIMO), single-input-single-output(SISO), single-input-multiple-output (SIMO), and/ormultiple-input-single-output (MISO) schemes. Moreover, the DAS 100 canbe configured to support multiple wireless air interfaces and/or tosupport multiple wireless operators.

The DAS 100 can use either digital transport, analog transport, orcombinations of digital and analog transport for communicating betweenthe head-end unit 104 and the remote units 102 a-d. The examples shownin FIGS. 2-8 are described below, for the purposes of illustration, asbeing implemented using digital transport. However, it is to beunderstood that such examples can also be implemented in DASs that useanalog transport or combinations of analog and digital transport.

In embodiments where the DAS 100 uses digital transport forcommunicating between the head-end unit 104 and the remote units 102a-d, digital samples indicative of the original wireless signals arecommunicated between the head-end unit 104 and the remote units 102 a-d.In such embodiments, the digital samples can be in the form of digitalin-phase (I) and quadrature (Q) samples (though it is to be understoodthat other embodiments can use other forms of digital samples). DigitalIQ samples can be produced from an analog wireless signal received atradio frequency (RF) by down-converting the received signal to anintermediate frequency (IF) or to baseband, digitizing thedown-converted signal to produce real digital samples, and digitallydown-converting the real digital samples to produce digital in-phase andquadrature samples. These digital IQ samples can also be filtered,amplified, attenuated, and/or re-sampled or decimated to a lower samplerate. The digital samples can be produced in other ways. The portion ofwireless spectrum can include, for example, a band of wireless spectrum,a sub-band of a given band of wireless spectrum, or an individualwireless carrier. Likewise, an analog wireless signal can be producedfrom digital IQ samples by digitally up-converting the digital IQsamples to produce real digital samples, performing a digital-to-analogprocess on the real samples in order to produce an IF or baseband analogsignal, and up-converting the IF or baseband analog signal to thedesired RF frequency. The digital IQ samples can also be filtered,amplified, attenuated, and/or re-sampled or interpolated to a highersample rate. The analog signal can be produced in other ways (forexample, where the digital IQ samples are provided to a quadraturedigital-to-analog converter that directly produces the analog IF orbaseband signal).

In embodiments where digital IQ samples are used for digital transport,the head-end unit 104 can be configured to generate one or more downlinkstreams of digital IQ samples from one or more signals or inputs thatare provided to the head-end unit 104 from the one or more base stations106. In such embodiments, each remote unit 102 a-d receives one or moredownlink streams of digital IQ samples produced by the head-end unit 104and generates one or more analog downlink radio frequency wirelesssignals that are radiated from one or more antennas that are associatedwith the remote unit 102 a-d. Typically, each downlink stream of digitalIQ samples is provided to a group of one or more remote units 102 a-d inorder to simulcast the generated analog downlink radio frequencywireless signals from multiple locations.

In embodiments where digital IQ samples are used for digital transport,each remote unit 102 a-d is configured to generate one or more uplinkstreams of digital IQ samples from one or more analog uplink radiofrequency wireless signals received by one or more antennas associatedwith the remote unit 102 a-d. For each downlink stream of digital IQsamples that is provided from the head-end unit 104 to a respectivegroup of one or more remote units 102 a-d, the head-end unit 104produces a corresponding uplink stream of digital IQ samples. Each suchuplink stream is typically produced by combining individual uplinkstreams of digital IQ samples from each of the one or more multipleremote units 102 a-d (for example, by digitally summing corresponding IQsamples from each uplink stream). The head-end unit 104 then uses eachsuch combined uplink stream of digital IQ samples to generate a signalor other output that is provided from the head-end unit 104 to a basestation 106.

In the examples shown in FIGS. 2-8, for the purposes of illustration,the same downlink signals are simulcast from all of the remote unitsand, accordingly, the corresponding uplink signals from all of theremote units are combined together by one or more summers. However, itis to be understood that other embodiments can be implemented in otherways (for example, different downlink signals can be provided todifferent groups of remote units, in which case the corresponding uplinksignals received from each of the different groups of remote units wouldbe separately combined together).

The analog downlink signals transmitted by the remote units can createintermodulation byproducts that, in a DAS with no duplexer or outputfilter, can feed back into the receive antennas of the remote units. Dueto limited transmitter/receiver isolation, the intermodulationbyproducts can show up in the receive/uplink path being transmitted tothe base station. The feedback intermodulation byproducts can interferewith uplink signals being received by the remote units. In a DAS systemwhere multiple remote units are combined, the receiver band noise andintermodulation byproducts from the transmitter of the remote unit canbe summed together with uplink signals from each remote unit. If thereceive signals are uncorrelated, each doubling of the number of remoteunits can add another three decibels of intermodulation byproduct noiseto the interference, reducing the overall signal to interference andnoise ratio (SINR). If the receive signals are correlated, then adoubling of the number of remote units can add another six decibels tothe intermodulation noise. It is desirable to reduce, attenuate,eliminate, or cancel the received intermodulation byproducts in order toincrease the SINR of the overall DAS.

To reduce the impact of the intermodulation byproducts on the receiveband, certain aspects and features relate to an intermodulation cancelerthat can cancel, reduce, attenuate, or eliminate the intermodulationbyproducts present in uplink signals on the receive path. As thedownlink signal transmitted by the base station is repeated and copiedby the head-end unit, the undesired intermodulation byproducts aresimilar. Due to the shouldering characteristics of spread-signalintermodulation, the transmit intermodulation byproducts tend to beclustered at one end of the receive band, such that the intermodulationsignals can share the same phase variation across frequency.Anti-correlation can be achieved by using a gain and phase adjustment toeach received signal. If the received signals vary significantly inphase and gain across the intermodulation byproducts, aspects andfeatures disclose a frequency-dependent transfer function to manipulatethe signal in order to cancel, reduce, attenuate, or eliminate theunwanted noise.

Unwanted intermodulation byproducts in the receive band can be canceled,reduced, attenuated, or eliminated by a processing device in thehead-end unit. The head-end unit can receive uplink signals transmittedby remote units, each uplink signal including unwanted intermodulationbyproducts. A processing device in the head-end unit can apply atransfer function to the received uplink signals in order to cancel out,reduce, attenuate, or eliminate the unwanted intermodulation byproducts.A transfer function can include the filtering used to modify theamplitude and gain of an input signal so that undesired intermodulationbyproducts are canceled, reduced, attenuated, or eliminated from theinput signal while desired uplink signals to be provided to the basestation are unmodified. Various methods exist for calculating thetransfer function to apply to received signals at the head-end unit.Such processing can be performed in other nodes in the DAS.

The examples shown in FIG. 2-8 are initially described below as beingimplemented in a digital DAS where digital transport is used tocommunicate digital samples indicative of original wireless signalsbetween the head-end units and remote units (perhaps via one or moreexpansion units or other intermediary units). The digital samples can bein the form of digital IQ data. In each of the examples shown in FIGS.2-8, one or more DAS nodes (for example, one or more head-end units,expansion or intermediary units, or remote units) includes one or moreprocessing devices or circuits 122 (collectively referred to here as a“processing device” 122) that digitally processes the digital IQ datathat is communicated in the DAS. The processing device 122 can beimplemented, for example, using one or more digital devices or circuitsthat perform digital signal processing (for example, using one or moredigital signal processors or other programmable processors and/or usingone or more field programmable gate arrays (FPGAs)). Each processingdevice 122 can be implemented in other ways.

In each of the examples shown in FIGS. 2-8, each processing device 122implements one or more transfer function filters 124. Each transferfunction filter 124 implements a variable filter that can be configuredto apply a calculated transfer function to the respective input of thetransfer function filter 124, the result of which is the output of thatfilter 124. Each processing device 122 also implements one or moresummers 126. Each summer 126 sums the multiple inputs to that summer 126(for example, by digitally summing corresponding IQ samples from thevarious inputs) to produce its output.

Each processing device 122 also implements one or more samplers 128.Each sampler 128 captures samples of its input while passing its inputthrough unchanged as the output of the sampler 128. The captured samplesare provided to one or more transfer function calculators 130. Eachprocessing device 202 implements one or more transfer functioncalculators 130. Each transfer function calculator 130 determines atransfer function based on the various samples it receives. Thecalculated transfer function can then be provided to one or moretransfer function filters 124 in order to configure the variable filterin each such transfer function filter 124 so that it applies thecalculated transfer function to the input of that transfer function 124.

The processing device 122 also implements one or more standard DASprocessing blocks 132 that perform standard digital DAS processing onthe downlink and uplink IQ data (for example, as described above inconnection with paragraphs [0013]-[0015]).

FIG. 2 is a flow diagram illustrating one exemplary method 200 forcalculating a transfer function designed to cancel, reduce, attenuate,or eliminate unwanted intermodulation byproducts received at thehead-end unit. A remote unit can be configured to generate and transmita test reference signal (block 202). The test reference signal can betransmitted during times of low user activity in the DAS or during a DASmaintenance period so as not to interfere with normal DAS operations. Areceive antenna at the remote unit can receive feedback signals from thetransmitted test reference signal (block 204), the feedback signalsincluding the unwanted intermodulation byproducts. A processing device122 in one or more nodes of the DAS can use the received feedback signalto determine an intermodulation byproduct model by comparing signalcharacteristics of the transmitted test signal and the received feedbacksignal (block 206). The differences in the signal characteristicsbetween the transmitted test signal and the received feedback signal cancorrespond to the intermodulation byproduct model (e.g., the amount ofamplitude and phase differences present in the receive signals). Forexample, the feedback signal can be provided to the head-end unit, and aprocessing device 122 in the head-end unit can use the received feedbacksignal to determine an intermodulation byproduct model by comparingsignal characteristics of the transmitted test signal and the receivedfeedback signal.

The processing device 122 (more specifically, one or more transferfunction calculators 130 implemented using the processing device 122)can then generate a transfer function that includes the inverse signalcharacteristics (e.g., inverse amplitude and phase) of theintermodulation byproduct model (block 208). During subsequent operationof the DAS, a processing device 122 in one or more nodes of the DAS(such as one or more of the head-end units, remote units, or anyexpansion units or other intermediary nodes) can apply the transferfunction to at least one DAS signal in order to cancel, reduce,attenuate, or eliminate the unwanted intermodulation byproducts (block210). For example, the transfer function can be applied to at least oneof the uplink signals communicated from the remote units to the head endunit or at least one of the downlink signals communicated from the headend units to the remote units.

In the case where the transfer function is applied to the uplink signalsreceived at the head end from the remote units, the transfer function isapplied to cancel, reduce, attenuate, or eliminate the unwantedintermodulation byproducts in the summed signals provided to the basestation. Applying the transfer function to received signals can include,for example, filtering the received signal with the filteringcharacteristics specified by the transfer function. The transferfunction can be applied, for example, prior to summing the receivedsignals from each remote unit (as shown in FIGS. 3-6).

In other aspects, the head-end unit can apply the transfer function toeach of the transmit signals being provided from the head-end unit toeach remote unit (shown in FIG. 7).

The transfer function can be applied to either the receive signals ortransmit signals in pairs, in small groups, or to all transmit signalsor all receive signals simultaneously.

Other methods to calculate the transfer function are possible. Forexample, the test reference signal can be a pre-determined referencesignal based on previously stored intermodulation byproduct models. TheDAS system operator can configure the DAS with the pre-determinedreference signal.

In another example, a remote unit can generate a pilot signal at theedge of the transmit band to allow the head-end unit to determine thetransfer function (at least in part due to antenna isolation variance).In this case, the transfer function can be updated during the normaloperation of the DAS.

In another method, the undesired intermodulation byproducts can bereduced without the use of a test reference signal. For example, thetransfer function can be determined by applying signal processingtechniques to minimize the interference signal in the uplink receivedsignal. In one aspect, the head-end unit can minimize like signals inthe desired uplink band, as the desired signals may bedifferent/non-correlated at each remote unit, while the undesiredtransmit intermodulation can be the same at each receiver. For example,if transmit signals from non-adjacent remote units are summed together,the desired signals from each remote unit can be non-correlated whilethe undesired intermodulation byproducts may be similar and cancellable.

In another aspect, the desired signals from multiple remote units may becommon or otherwise correlated (e.g., if two adjacent remote unitsreceive uplink signals from a single mobile device). In this aspect, thehead-end unit may determine, for example, in which locations in the bandthe undesired signals may fall (from the original transmit signals) anddetermine the cancellation by minimizing signal/noise in the bandwidththat has the undesired signals.

As shown in FIG. 3, the head-end unit 300 can include multiple transferfunction filters 124, each transfer function filter 124 receiving uplinksignals from a different respective remote unit. The received signalscan be provided to a summer 126 included in the head-end unit 300. Asampler 128 in the head-end unit 300 can then sample the summed signals,generating sampled signals provided to a transfer function calculator130. The transfer function calculator 130 can determine the transferfunctions for each transfer function filter 124. During subsequentreceived signals, each transfer function filter 124 can apply thetransfer function calculated by the transfer function calculator 130,cancelling, reducing, attenuating, or eliminating the unwantedintermodulation byproducts before the uplink signals are summed by thesummer 126.

FIGS. 4-6 depict additional configurations for a head-end unit forcancelling, reducing, attenuating, or eliminating unwantedintermodulation byproducts according to certain aspects. FIG. 4 depictsa head-end unit 400 configured such that summation and cancellation,reduction, attenuation, or elimination of the unwanted intermodulationbyproducts can be performed in small groups of remote unit inputs (e.g.,two or three per group). The minimized signals can then be summedtogether at a summer 126 before being provided to a DAS signalprocessing block 132. The DAS signal processing block 132 can thenperform standard digital DAS processing on the downlink and uplink IQdata (for example, as described above in connection with paragraphs[0013]-[0015]).

In other examples, the transfer function may be applied to n−1 of thetotal number n of received signals being summed, since the cancellationoccurs between/amongst the summed signals. FIG. 5 depicts a blockdiagram of a head-end unit 500 calculating and applying a transferfunction to n−1 of the total number of received signals being summed.The head-end unit 500 can include multiple transfer function filter 124,each transfer function filter 124 receiving uplink signals from adifferent respective remote unit. The received signals can be providedto a summer 126 included in the head-end unit 500. A sampler 128 in thehead-end unit 500 can then sample the summed signals, generating sampledsignals provided to a transfer function calculator 130. The transferfunction calculator 130 can determine the transfer functions for eachtransfer function filter 124. During subsequent received signals, eachtransfer function filter 124 can apply the transfer function calculatedby the transfer function calculator 130, cancelling, reducing,attenuating, or eliminating the unwanted intermodulation byproductsbefore the uplink signals are summed by the summer 126.

FIG. 6 depicts a head-end unit 600 configured such that each receivedsignal from the respective remote units is provided to a respectivesampler 128. The samples of the individual received signals can beprovided to the transfer function calculator 130. The transfer functioncalculator 130 can extract the reference signals communicated in eachreceived signal (by processing the associated digital IQ data) in orderto determine the individual transfer functions/non-linear models of thereceive isolation path through the remote units. The transfer functioncalculator 130 provides the calculated transfer functions to therespective transfer function filters 124, which can apply the transferfunctions to the received uplink signals from each remote unit.Undesired intermodulation byproducts can be canceled, reduced,attenuated, or eliminated as they are being summed by the summer 126.

As noted above, in other aspects, the head-end unit can apply thetransfer function to each of the transmit signals being provided fromthe head-end unit to each remote unit. FIG. 7 depicts one example of ahead-end unit 700 configured so that a downlink signal provided to eachremote unit is filtered by a transfer function filter 124. The transferfunction calculator 130 calculates the transfer functions that areapplied to the transfer functions 124 based on samples made by samplers128 in the uplink signal path.

These techniques can be used in other DAS nodes. For example, as shownin FIG. 8, a remote unit 800 includes a processing device 122 that canbe configured such that each received signal transmitted from thatremote unit to the head-end unit (either directly or via an expansionunit or other intermediary unit) is first sampled by a sampler 128. Thesamples of the received signal can be provided to a transfer functioncalculator 130 in the remote node 800. The transfer function calculator130 can extract the reference signals communicated in each receivedsignal (by processing the associated digital IQ data) in order todetermine the transfer function/non-linear model of the receiveisolation path through the remote unit. The transfer function calculator130 provides the calculated transfer function to a transfer functionfilter 124, which can apply the transfer function to the received uplinksignal, thereby cancelling, reducing, attenuating, or eliminating theundesired intermodulation byproducts before the uplink signal istransmitted from the remote unit to the head-end unit.

The techniques described here can also be applied in other nodes orparts of a DAS (for example, in expansion units or other intermediaryunits).

The techniques described above can also be used in a DAS that makes useof analog transport or combinations of analog and digital transport. Inone example of a DAS that uses analog transport, a node can beconfigured to digitize the analog signals to produce digital IQ datafrom the analog signals (for example, in the manner described above).The resulting digital IQ data then be used to calculate and apply atransfer function (using, for example, digital signal processingtechniques similar to the ones described above) in order to cancel,reduce, attenuate, or eliminate undesired intermodulation byproducts.The resulting processed digital IQ data can then be used to producesuitable analog signals, which can then be transported as inconventional analog DAS systems.

These illustrative examples are given to introduce the reader to thegeneral subject matter discussed here and are not intended to limit thescope of the disclosed concepts. The following sections describe variousadditional aspects and examples with reference to the drawings in whichlike numerals indicate like elements, and directional descriptions areused to describe the illustrative examples but, like the illustrativeexamples, should not be used to limit the present disclosure.

EXAMPLE EMBODIMENTS

Example 1 includes a distributed antenna system (DAS) comprising: aplurality of nodes, including: a head-end unit; and a plurality ofremote units that are communicatively coupled to the head-end unit;wherein the head-end unit is configured to receive uplink receivedsignals from remote units that wirelessly transceive signals in acoverage area; wherein the head-end unit is configured to sum two ormore of the uplink received signals to produce a summed uplink receivedsignal; and wherein at least one of the nodes of the DAS includes aprocessing device configured to determine a transfer function and applythe transfer function to signals in the DAS to cancel, reduce,attenuate, or eliminate intermodulation byproducts in the summed uplinkreceived signal.

Example 2 includes the DAS of Example 1, wherein the processing deviceis configured to determine the transfer function by comparing a testreference signal transmitted by a remote unit to a receive feedbacksignal to determine inverse signal characteristics of unwantedintermodulation byproducts in the receive feedback signal.

Example 3 includes the DAS of Example 2, wherein the inverse signalcharacteristics include a phase shift and an amplitude shift.

Example 4 includes any of the DASs of Examples 1-3, wherein the transferfunction is determined by minimizing the uplink received signal afterthe uplink received signals are summed to produce the summed uplinkreceived signal.

Example 5 includes any of the DASs of Examples 1-4, wherein the transferfunction is determined by minimizing the uplink received signals in aspecific portion of an uplink band determined to have intermodulationnoise from analysis of the downlink transmitted signals.

Example 6 includes any of the DASs of Examples 1-5, wherein the transferfunction is applied to the uplink received signals prior to summation.

Example 7 includes any of the DASs of Examples 1-6, wherein the transferfunction is applied to one or more of the uplink received signals in thehead-end unit.

Example 8 includes any of the DASs of Examples 1-7, wherein the transferfunction is applied to one or more of the signals in at least one of theremote units.

Example 9 includes any of the DASs of Examples 1-8, wherein the transferfunction is applied to one or more of the signals in an expansion unitused to communicatively couple at least one remote unit to the head-endunit.

Example 10 includes any of the DASs of Examples 1-9, wherein thetransfer function is applied in at least one node of the DAS to at leastone transmit signal communicated over the DAS.

Example 11 includes a method performed in a distributed antenna system(DAS) comprising a plurality of nodes, including a head-end unitcommunicatively coupled to a plurality of remote units, the methodcomprising: transmitting a test reference signal from the remote unit;receiving a feedback signal from the test reference signal at a receiveantenna of the remote unit; calculating, from the test reference signaland the feedback signal, a transfer function to cancel, reduce,attenuate, or eliminate intermodulation byproducts in subsequent signalsreceived by the head-end unit; and applying the transfer function tosignals of the DAS in order to cancel, reduce, attenuate, or eliminateintermodulation byproducts.

Example 12 includes the method of Example 11, further comprisingproviding the test reference signal and the feedback signal to thehead-end unit for use in calculating the transfer function to cancel,reduce, attenuate, or eliminate intermodulation byproducts in subsequentsignals received by the head-end unit.

Example 13 includes any of the methods of Examples 11-12, whereincalculating, from the test reference signal and the feedback signal, thetransfer function to cancel, reduce, attenuate, or eliminateintermodulation byproducts in subsequent signals received by thehead-end unit comprises: comparing the test reference signal to thereceive feedback signal to determine inverse signal characteristics ofunwanted intermodulation byproducts in the feedback signal.

Example 14 includes the method of Example 13, wherein the inverse signalcharacteristics include a phase shift and an amplitude shift.

Example 15 includes any of the methods of Examples 11-14, wherein thetransfer function is calculated by minimizing the uplink received signalafter two or more uplink received signals are summed to produce a summeduplink received signal.

Example 16 includes any of the methods of Examples 11-15, whereincalculating, from the test reference signal and the feedback signal, thetransfer function to cancel, reduce, attenuate, or eliminateintermodulation byproducts in subsequent signals received by thehead-end unit comprises: minimizing uplink received signals in aspecific portion of an uplink band determined to have intermodulationnoise from analysis of the downlink transmitted signals.

Example 17 includes any of the methods of Examples 11-16, wherein thetransfer function is applied to two or more uplink received signalsprior to summation.

Example 18 includes any of the methods of Examples 11-17, wherein thetransfer function is applied to one or more of uplink received signalsin the head-end unit.

Example 19 includes any of the methods of Examples 11-18, wherein thetransfer function is applied to one or more of signals in at least oneof the remote units.

Example 20 includes any of the methods of Examples 11-19, wherein thetransfer function is applied to one or more signals in an expansion unitused to communicatively couple at least one remote unit to the head-endunit.

Example 21 includes any of the methods of Examples 11-20, wherein thetransfer function is applied in at least one node of the DAS to at leastone transmit signal communicated over the DAS.

What is claimed is:
 1. A distributed antenna system (DAS) comprising: a plurality of nodes, including: a head-end unit; and a plurality of remote units that are communicatively coupled to the head-end unit; wherein the head-end unit is configured to receive uplink received signals from remote units that wirelessly transceive signals in a coverage area; wherein the head-end unit is configured to sum two or more of the uplink received signals to produce a summed uplink received signal; and wherein at least one of the nodes of the DAS includes a transfer function filter and a processing device configured to: determine a transfer function; and configure a variable filter of the transfer function filter to apply the determined transfer function to signals in the DAS, wherein application of the transfer function cancels, reduces, attenuates, or eliminates intermodulation byproducts in the summed uplink received signal.
 2. The DAS of claim 1, wherein the processing device is configured to determine the transfer function by comparing a test reference signal transmitted by a remote unit to a receive feedback signal to determine inverse signal characteristics of unwanted intermodulation byproducts in the receive feedback signal.
 3. The DAS of claim 2, wherein the inverse signal characteristics include a phase shift and an amplitude shift.
 4. The DAS of claim 1, wherein the transfer function is determined by minimizing the uplink received signals after the uplink received signals are summed to produce the summed uplink received signal.
 5. The DAS of claim 1, wherein the plurality of remote units is configured to transmit downlink signals, wherein the transfer function is determined by minimizing the uplink received signals in a specific portion of an uplink band determined to have intermodulation noise from analysis of the transmitted downlink signals.
 6. The DAS of claim 1, wherein the transfer function is applied to the uplink received signals prior to summation.
 7. The DAS of claim 1, wherein the transfer function is applied to one or more of the uplink received signals in the head-end unit.
 8. The DAS of claim 1, wherein the transfer function is applied to one or more of the signals in at least one of the remote units.
 9. The DAS of claim 1, wherein the transfer function is applied to one or more of the signals in an expansion unit used to communicatively couple at least one remote unit to the head-end unit.
 10. The DAS of claim 1, wherein the transfer function is applied in at least one node of the DAS to at least one transmit signal communicated over the DAS.
 11. A method performed in a distributed antenna system (DAS) comprising a plurality of nodes, including a head-end unit communicatively coupled to a plurality of remote units, the method comprising: transmitting a test reference signal from the remote unit; receiving a feedback signal from the test reference signal at a receive antenna of the remote unit; calculating, from the test reference signal and the feedback signal, a transfer function to cancel, reduce, attenuate, or eliminate intermodulation byproducts in subsequent signals received by the head-end unit; and configuring a variable filter of a transfer function filter to apply the calculated transfer function to signals of the DAS, wherein application of the calculated transfer function cancels, reduces, attenuates, or eliminates intermodulation byproducts.
 12. The method of claim 11, further comprising providing the test reference signal and the feedback signal to the head-end unit for use in calculating the transfer function to cancel, reduce, attenuate, or eliminate intermodulation byproducts in subsequent signals received by the head-end unit.
 13. The method of claim 11, wherein calculating, from the test reference signal and the feedback signal, the transfer function to cancel, reduce, attenuate, or eliminate intermodulation byproducts in subsequent signals received by the head-end unit comprises: comparing the test reference signal to the receive feedback signal to determine inverse signal characteristics of unwanted intermodulation byproducts in the feedback signal.
 14. The method of claim 13, wherein the inverse signal characteristics include a phase shift and an amplitude shift.
 15. The method of claim 11, wherein the transfer function is calculated by minimizing the uplink received signals after two or more uplink received signals are summed to produce a summed uplink received signal.
 16. The method of claim 11, further comprising transmitting downlink signals from the remote unit wherein calculating, from the test reference signal and the feedback signal, the transfer function to cancel, reduce, attenuate, or eliminate intermodulation byproducts in subsequent signals received by the head-end unit comprises: minimizing uplink received signals in a specific portion of an uplink band determined to have intermodulation noise from analysis of the transmitted downlink signals.
 17. The method of claim 11, wherein the transfer function is applied to two or more uplink received signals prior to summation.
 18. The method of claim 11, wherein the transfer function is applied to one or more of uplink received signals in the head-end unit.
 19. The method of claim 11, wherein the transfer function is applied to one or more of signals in at least one of the remote units.
 20. The method of claim 11, wherein the transfer function is applied to one or more signals in an expansion unit used to communicatively couple at least one remote unit to the head-end unit.
 21. The method of claim 11, wherein the transfer function is applied in at least one node of the DAS to at least one transmit signal communicated over the DAS. 